This invention relates to an acoustic diaphragm which is of the type using an organic sheet material as a substrate and is useful for loudspeakers.
In a loudspeaker utilizing an acoustic diaphragm as a sound-radiating means, attached to a voice coil which is operably positioned in a magnetic gap, the characteristic of the speaker primarily depends on the characteristic of the acoustic diaphragm. Loudspeakers are generally required to exhibit a high efficiency in converting an input energy into sound wave and have a flat frequency characteristic over a wide frequency range. To satisfy these requirements, an acoustic diaphragm for loudspeakers must have a small specific gravity, a large value for Young's modulus and an internal loss of an adequate scale. The specific gravity of the diaphragm greatly influences the electrical-to-acoustical energy conversion efficiency of a magnetic speaker: the smaller the specific gravity the higher the efficiency. A large Young's modulus (relative to specific gravity) and rather a large internal loss factor of the diaphragm lead to a flat frequency-output characteristic of the speaker particularly at high frequencies. It is not easy, however, to provide an acoustic diaphragm which meets these requirements all together since a diaphragm material featuring a small specific gravity generally has a small Young's modulus.
Paperboard has widely been used as the material of acoustic diaphragms with various treatments, but has not always been satisfactory in regard to the aforementioned physical properties. Particularly for tweeters, paperboard diaphragms have the disadvantage of hardly exhibiting a flat response at high frequencies due to their insufficient rigidity.
Thin metal sheet diaphragms such as of aluminum or titanium have been used particularly for tweeters to take the advantage of a large Young's modulus of such a metal relative to specific gravity. However, these metal diaphragms have excessively small internal loss factors and, hence, cannot easily be designed to exhibit a satisfactorily flat frequency-output characteristic. Besides, the use of a metal which has a greater specific gravity than, for example, paperboard causes a lowering of the efficiency of speakers.
A different type of acoustic diaphragms have been provided by utilizing a fabric sheet such as cotton cloth as the basic material of the diaphragms and coating and/or impregnating the fabric sheet with either natural rubber or a synthetic rubber. Speakers given by diaphragms of this type are fairly good in the flatness of the response in a medium frequency range but are unsatisfactory in the efficiency due to considerably large values for specific gravity of the diaphragms and, besides, are of little use as tweeters because of comparatively small values for Young's modulus of the diaphragms.